Many optoelectronic sensors operate according to the scanning principle, in which a light beam is transmitted into the monitoring area and the light beam remitted by objects is received again in order to electronically evaluate the reception signal. The light time of flight is often measured with a known phase or pulse method to determine the distance of the scanned object.
In order to extend the measuring region, on the one hand the scanning beam can be moved, as is done in a laser scanner. There, a light beam generated by a laser periodically scans the monitoring area by means of a deflection unit. In addition to the measured distance information, the angular position of the object is determined from the angular position of the deflection unit so that the position of the object in the monitoring area is detected in two-dimensional polar coordinates.
Another possibility for extending the measuring region is to detect a plurality of measurement points by a plurality of scanning beams. This can also be combined with a laser scanner, which in that case not only scans a monitoring plane, but a three-dimensional spatial area via a plurality of monitoring planes. In most laser scanners, the scanning movement is achieved by a rotary mirror. It is also known in the art, in particular when using a plurality of scanning beams, to rotate the entire measuring head with light transmitters and light receivers instead, as for example described in DE 197 57 849 B4.
In order to evaluate the remitted light of a plurality of scanning beams, the light receiver must be able to detect them individually. For this purpose, a receiving optics is provided in order to focus the scanning beams on defined receiving elements. A simple solution is to form the receiving optics as a single aspherical lens. However, if a plurality of scanning beams are to be detected which are at an angle to one another, a relatively large image field results, and a common single lens has large imaging errors in the off-axial image regions.
This can in principle be solved by each receiving element having its own lens. Then, only a very small image field needs to be imaged which a single lens easily can do. The overall arrangement with a large number of single lenses, however, again is very complex.
It is also known for a long time to use an objective of a plurality of lenses for imaging an extended field of view. Thus, a high imaging quality can be achieved, but the manufacturing and adjustment costs are too high, especially in a laser scanner where the moving mass is to remain as small as possible. What is needed is a solution with possibly only one lens.
Among the known single lenses, the so-called “Wollaston landscape lens” still has the best imaging properties for an extended image field. This is a convex-concave lens, i.e. an inwardly curved meniscus shape, and an aperture arranged with an offset. Such a lens tends to be a good compromise between cheap manufacturing of a single asphere and a high image quality of an objective. In photography, the image points in an imaging field of ±12°-±25° are imaged with sufficiently small imaging error with f-numbers (quotient of focal length and diameter of the entrance pupil) k=10 . . . 16. Such large f-numbers, i.e. small apertures, cannot be used in particular for time of flight measurements because too much light is lost. When the aperture is increased, the imaging errors are also increased.